Silent brain ischemia (SBI) is a condition of small ischemic injuries in the brain that are a side effect of medical procedures, particularly vascular procedures and surgeries. Ischemia can also occur in other organs. Any foreign or endogenous material that is accidentally released into the arterial circulation during a medical procedure may result in SBI or ischemia of other organs, such as diffuse embolic ischemia. SBI is a heterogeneous syndrome, in which the exact source of an embolus may vary. For example, an embolus or microembolus that causes SBI, or ischemia of other organs, may be comprised of diverse substances including bubbles, oil, fat, cholesterol, coagulated blood and/or debris. Usually, the ischemic injury is diffuse. In SBI, an area of the brain is showered by microemboli and individual foci are difficult to detect with conventional magnetic resonance imaging (MRI). For example, microembolic signals are found during the injection of contrast agent and during probing of vessels (Bendszus M and Stoll G, 5 Lancet Neurol. 364-372, 2006). The exact mechanism of the tissue damage in SBI is unknown (Bendszus M and Stoll G, 5 Lancet Neurol. 364-372, 2006).
Due to the diffuse nature of the injury, a patient with SBI usually lacks focal, clearly defined neurological deficits as in stroke. However, behavioral changes, neuropsychological deficits and aggravated vascular dementia are frequently observed in a large number of patients after major surgery (e.g. coronary bypass, valve replacement surgery, carotid endarterectomy or stenting) and in many patients with vascular interventions, coronary angiography, arterial lines or intra-aortic counterpulsation devices in intensive care units. These symptoms are extremely common after medical procedures, but are rarely seen as directly related to the procedure. With the advent of increasingly sensitive imaging modalities such as diffusion-weighted MRI (DWI), there has been an increasing awareness of injuries to the brain that present without overt clinical symptoms such as paralysis or sensitivity defects.
Up to 45% of patients who have had surgeries and procedures develop SBI, especially those that involve the heart and vascular structures. Coronary angiography, performed over two million times annually in the USA, has a risk of SBI of 11-15% (Bendszus M and Stoll G, 5 Lancet Neurol. 364-372, 2006). Up to 26% of patients that underwent diagnostic angiography, up to 54% of patients that underwent carotid artery stenting and up to 45% of patients after heart surgery may be afflicted with SBI (Bendszus M and Stoll G, 5 Lancet Neurol. 364-372, 2006).
The clinical manifestations of SBI include behavioral and neuropsychological changes, in addition to increased risk for cognitive decline, increased risk of stroke and worsening dementia (Vermeer S E et al. 34 Stroke 1126-1129, 2003; Kobayashi S et al. 28 Stroke 1932-1939, 1997). The presence of SBI has been shown to more than double the risk of dementia in patients 60-90 years of age, and results in a steeper decline in global cognitive function and worse performance on neuropsychological testing (Vermeer S E et al. 34 Stroke 1126-1129, 2003; Lopez O L et al. 60 Arch. Neurol. 1394-1399, 2003).
SBI is distinct from clinical stroke. Clinical stroke leads to clearly defined neurological deficits, and often either involves the spontaneous rupture of a vulnerable plaque in an artery that supplies the brain with oxygenated blood or is due to thrombembolism from the heart caused by atrial fibrillation. In contrast to stroke, patients who are at risk for SBI may not have underlying atherosclerotic disease or risk factors for stroke or thrombosis. Patients that undergo a vascular procedure may include, for example, a patient with a congenital heart defect. Such patients, who are otherwise healthy, are still at risk for SBI as a side effect of the microemboli introduced during a medical procedure (Harrison's Principles of Internal Medicine 16th Ed. (Kasper D L, Fauci, A S, Longo, D L, Braunwald E, Hauser S L, Jameson J L eds., 2005)). Therefore the patient population that would benefit from a preventative therapy or treatment of SBI includes patients undergoing a medical procedure involving contact with structures of the vascular system.
Although heparin has been shown to reduce clinically silent embolic events caused by intra-arterial cerebral angiography (Bendszus et al., Circulation 110:2110-2115, 2004), it carries a relatively high risk of extra- and intracranial bleeding complications. Moreover, it is not clear if different SBI events, caused by different types of emboli, involve similar molecular mechanisms and/or would be treatable using similar therapeutics. A desired therapy would reduce ischemic injury caused by all types of emboli, including microemboli comprised of bubbles, oil, fat, cholesterol, coagulated blood and/or debris, but would not affect hemostasis such as heparin does. Accordingly, an effective and safe treatment for SBI and ischemia in other organs, such as diffuse embolic ischemia, irrespective of the cause, is needed. The development of an animal model to study SBI and evaluate treatments has been challenging, since diffuse micro-injuries need to be produced without causing overt stroke. Thus, realistic animal models of SBI are still needed to study the molecular mechanisms of tissue damage and to evaluate therapeutic candidates.
Factor XII (FXII) is a serine protease that is involved in the activation of the intrinsic coagulation cascade. Recently, it was found that deficiency or inhibition of Factor XII in mice reduced brain damage in stroke models and was protective against arterial thrombus formation, but without increasing the risk of bleeding (WO 2006/066878; WO 2008/098720; Kleinschnitz C et al. 203 J. Exp. Med. 513-518, 2006; Renne T et al. 202 J. Exp. Med. 271-281, 2005). Similar to FXII deficient mice, humans that are deficient in FXII do not suffer from abnormal bleeding diathesis, even during major surgical procedures (Ratnoff O D and Colopy J E, 34 J. Clin. Invest. 602-613, 1955; Colman R W, Hemostasis and Thrombosis. Basic Principles & Clinical Practice 103-122 (Colman R W, Hirsch J, Mader V J, Clowes A W, George J eds., Lippincott Williams & Wilkins, Philadelphia, 2001); Schmaier A H, 118 J. Clin. Invest. 3006-3009, 2008).
Recently, Infestin-4 was reported to be a novel inhibitor of activated FXII (FXIIa). Infestins are a class of serine protease inhibitors derived from the midgut of the hematophagous insect, Triatoma infestans, a major vector for the parasite Trypanosoma cruzi, known to cause Chagas' disease (Campos I T N et al. 32 Insect Biochem. Mol. Bio. 991-997, 2002; Campos I T N et al. 577 FEBS Lett. 512-516, 2004). This insect uses these inhibitors to prevent coagulation of ingested blood. The Infestin gene encodes 4 domains that result in proteins that can inhibit different factors in the coagulation pathway. In particular, domain 4 encodes a protein (Infestin-4) that is a strong inhibitor of FXIIa. Infestin-4 has been administered in mice without bleeding complications (WO 2008/098720).
Despite the heterogeneous mechanisms leading to SBI and ischemia in other organs, including diffuse embolic ischemia, the embodiments of this application provide inhibitors of FXII, particularly proteins comprising Infestin-4, and variants thereof, to treat SBI and ischemia in other organs. Further, the application provides animal models of SBI that mimic various types of emboli that may enter into the circulation during a medical procedure. The animal models described herein may be useful as tools to study SBI and evaluate therapeutic candidates.